This invention relates to improved thermal energy storage systems and more particularly to systems in which portions of a stored liquid such as water are removed from a storage tank, externally heated, and reintroduced into the tank in a manner to establish and maintain a high degree of thermal stratification in the tank.
Flow tubes or pipe arrangements for maintaining zones at different temperature within a liquid storage tank are known, as described, for example, in U.S. Pat. Nos. 4,253,446; 4,146,087; and 4,340,033. In the first of these there is disclosed a domestic hot water storage tank having a wall dividing the tank into an upper, high temperature portion and a lower, low temperature portion. The low temperature portion contains a heat exchanger receiving heated fluid from a solar collector, and the upper portion contains one or more auxiliary heating devices. Controlled flow of water from one portion to another is made possible by two orifices in different quadrants of the divider wall. In U.S. Pat. No. 4,146,087 a fluid accumulation tank is described which includes a vertical ascension tube whose bottom end receives heated fluid and which has curved nozzles at selected levels to direct ascending heated fluid into the tank at levels having nearly the same temperature as the heated fluid. U.S. Pat. No. 4,340,033 discloses a liquid storage tank in which stratification is promoted by means of a temperature-operated valve which directs returning water to either of two different levels of the tank where it enters through velocity reducers consisting of enlarged horizontal pipes whose ends are truncated at an angle.
Known fluid storage systems such as those set forth in the above-referenced patents have a number of deficiencies. For example, the thermal stratification achievable in the two-zone tank system of U.S. Pat. No. 4,253,446 is rather limited, particularly since each zone includes a heating element near the bottom thereof and no device for guiding heated fluid to levels having similar fluid temperatures. Multi-zone systems utilizing valve controls are rather complex and expensive. Systems which heat a storage fluid external to a tank and have stratification promoters such as simple ascension tubes generally lack means for distributing returning fluid to either a higher or a lower level as warranted by its temperature. Known systems also lack effective measures for reducing the momentum of incoming fluid to minimize fluid mixing in a fluid storage tank. Also, prior art fluid storage systems whose primary application is the heating of potable water by heat exchange with a separate fluid such as a solar collector fluid generally do not provide efficient heat transfer while at the same time adequately separating water to be heated from the solar collector fluid, which may be toxic.
Accordingly, it is an object of the invention to provide an improved thermal energy storage system.
It is an object of the invention to provide a liquid storage tank containing simple, yet effective means for maintaining a high degree of thermal stratification.
It is an object of the invention to provide a liquid storage system wherein portions of the liquid removed from a storage tank are heated and reintroduced into the tank at levels having nearly the same temperature as that of the reintroduced liquid.
It is also an object of the invention to provide a liquid storage tank wherein the momentum of heated liquid reintroduced into the tank is reduced to low levels in order to minimize mixing of fluid in the tank.
It is also an object of the invention to provide a storage system for potable water which in addition to achieving the above objects, attains high heat transfer and double separation between the working fluid of a heater and potable water withdrawn from a storage tank for heating by the working fluid.